The present disclosure is related to a stir-transport member which stirs and transports powder such as a developer, a development device provided therewith, and an image forming apparatus incorporating the development device.
In an image forming apparatus, a latent image formed on an image bearing member which includes a photosensitive body and the like is developed by a development device to thereby visualize the latent image as a toner image. An example of such a development device adopts a two-component developing method in which a two-component developer is used. This type of development device stores, in a development container, a developer including a carrier and toner. The development device includes a development roller which supplies the developer to an image bearing member, and a stir-transport member which supplies the developer in the development container to the development roller by stirring and transporting the developer.
With the two-component developing method, insufficiently charged toner may result in an defective image suffering from scattered toner, a fog phenomenon, or the like. To prevent this, it is necessary to fully stir the toner and the carrier, to thereby charge the toner to a predetermined level.
To achieve this, for example, there has been known a powder stir-transport member that includes: a shaft member; a main transport blade which transports powder in a first direction toward one end side along the axis of the shaft member as the shaft member rotates; and sub transport means which generates a transport movement of transporting part of the powder in a second direction toward the other end side along the axis of the shaft member as the shaft member rotates. As the sub transport means, there has been known an oppositely-wound (opposite-phased) sub transport blade which is smaller than the main transport blade in diameter.
With this arrangement, by the sub transport means generating the transport movement in the direction opposite to the direction in which the powder is transported by the main transport blade, convection is caused to occur with respect to part of the powder while it is transported, and this makes it possible to promote the stirring of the powder without obstructing the transporting operation performed by the main transport blade.
On the other hand, there has been known a resin toner-stir-and-transport screw that has a spiral blade whose section taken along a direction that crosses the longitudinal direction thereof has a trapezoidal shape, and in which, in a turn thereof around a shaft, there are formed a plurality of portions in each of which a part corresponding to the bottom of the trapezoidal shape is more swollen than in the other portions.
With this arrangement, provision of the portions in each of which a part corresponding to the bottom of the trapezoidal shape is more swollen than in the other portions helps enhance the effect of strengthening the spiral blade, and thus to achieve improved performance of stirring and transporting toner. In addition, the volume of the spiral blade can be made smaller than with a design in which parts corresponding to the bottom of the trapezoidal shape are formed wider all along the spiral blade, and this makes it possible to widen the space in each pitch (where toner can be held) of the spiral blade. Furthermore, by swelling portions corresponding to joints between separate molds, the toner-stir-and-transport screw can be easily pulled out from the mold.
In the above-described stir-transport member provided with the main transport blade and the sub transport blade, if, as described above, portions in each of which a part corresponding to the bottom of the trapezoidal shape is more swollen than in the other portions are formed in a turn of a spiral blade around the shaft, it is necessary to form such portions not only in the main transport blade but also in the sub transport blade.
This results in an arrangement where each spiral blade includes portions in each of which a part corresponding to the bottom of the trapezoidal shape is swollen. Such swell portions are inferior in transporting performance to the other portions, and moreover, are liable to change the flow of the developer and ruffle the developer. Thus, it is not desirable to form many swell portions. Also, in addition to the portions in each of which a part corresponding to the bottom of the trapezoidal shape is swollen, portions at which the main transport blade and the sub transport blade cross each other also tend to change the flow of the developer and ruffle the developer.
Thus, at the swell portions of the main and sub transport blades and at the portions where the main transport blade and the sub transport blade cross each other, the developer flows in an unstable manner. As a result, the developer receives an increased amount of stress while it is stirred and transported between the stir-transport member and the housing of the development device, and this disadvantageously promotes degradation of the carrier. This degradation of the carrier is liable to be affected by change of flowability of the developer which is caused by an environmental change, and the degradation becomes remarkable when the flowability of the developer is reduced.